Notice: This Wiki is now read only and edits are no longer possible. Please see: https://gitlab.eclipse.org/eclipsefdn/helpdesk/-/wikis/Wiki-shutdown-plan for the plan.
Difference between revisions of "EclipseLink/Development/Dynamic/def1"
(→Mechanism in Java5/Java6) |
(→Mechanism in Java5/Java6) |
||
Line 14: | Line 14: | ||
[[Image:custom_classloader.gif]] | [[Image:custom_classloader.gif]] | ||
<br /> | <br /> | ||
+ | The basic implementation pattern is as follows - in the constructor, the newly-created <code>MyCustomClassLoader</code> is added to the | ||
+ | runtime inheritance-chain by passing the parent loader to <code>super</code>. | ||
<source lang=java5> | <source lang=java5> | ||
public class MyCustomClassLoader extends ClassLoader { | public class MyCustomClassLoader extends ClassLoader { |
Revision as of 15:40, 31 August 2009
Dynamic Persistence
Dynamic Persistence is defined as the ability to create a persistent entity class and use it within an application without a-priori the Java class existing (no .class file on the classpath or in the relevant .jar/.war archive).
Mechanism in Java5/Java6
In order to create a Java class at runtime without Java source code, the use of a custom ClassLoaderer is required, along with a bytecode manipulation framework (such as ASM or some other library framework).
Java classloaders form an inheritance-chain at run-time, with the system (Bootstrap, Extension and System) class loaders
strictly controlled by the JVM. Once an application is launched (via an Application loader), a new loader MyCustomClassLoader
can
be added to the chain.
The basic implementation pattern is as follows - in the constructor, the newly-created MyCustomClassLoader
is added to the
runtime inheritance-chain by passing the parent loader to super
.
public class MyCustomClassLoader extends ClassLoader { public MyCustomClassLoader (ClassLoader parent) { super(parent); } @Override protected Class<?> findClass(String className) throws ClassNotFoundException { if (some_condition) { try { byte[] bytes = call bytecode manipulation framework to generate a class; return defineClass(className, bytes, 0, bytes.length); } catch (ClassFormatError cfe) { throw new ClassNotFoundException(className, cfe); } } return super.findClass(className); } }
Mechanism in Java7
Future
JSR-292 (Supporting Dynamically Typed Languages on the Java ™ Platform) introduces a new type of classloader,
java.dyn.AnonymousClassLoader
. The AnonymousClassLoader
class is designed to solve two problems:
- Generating many classes with similar bytecode and only minor changes is very inefficient, wasting a lot of precious memory.
- Generated bytecode must be contained in a class, which must be contained in a ClassLoader, which keeps a hard reference to the class; as a result, to make even one byte of bytecode garbage-collectable, it must be wrapped in its own class and its own classloader.
First, classes loaded by AnonymousClassLoader
are not given full-fledged symbolic names in the global symbol tables; they're given rough numeric identifiers. They are effectively anonymized, allowing much more freedom to generate them at will, since naming conflicts essentially do not happen.
Second, the classes are loaded without a parent ClassLoader, so there's no overprotective mother keeping them on a short leash. When the last normal references to the class disappear, it's eligible for garbage collection like any other object.
Third, it provides a mechanism whereby an existing class can be loaded and slightly modified, producing a new class with those modifications but sharing the rest of its structure and data.